Fluorescence analyzer for lignin

ABSTRACT

A method and apparatus for measuring lignin concentration in a sample of wood pulp or black liquor comprises a light emitting arrangement for emitting an excitation light through optical fiber bundles into a probe which has an undiluted sensing end facing the sample. The excitation light causes the lignin concentration to produce fluorescent emission light which is then conveyed through the probe to analyzing equipment which measures the intensity of the emission light. Measures a 
     This invention was made with Government support under Contract Number DOE: DE-FC05-90CE40905 awarded by the Department of Energy (DOE). The Government has certain rights in this invention.

This invention was made with Government support under Contract NumberDOE: DE-FC05-90CE40905 awarded by the Department of Energy (DOE). TheGovernment has certain rights in this invention.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates in general to the pulp and paper industry,and in particular to a new and useful analyzer for monitoring theconcentration of lignin in wood pulp and black liquor.

In the pulp and paper industry, the production of paper productsrequires that lignin be partially removed from the wood chip feed stockprior to making paper products. Lignin is a polymer of complex chemicalstructure which "cements" the wood's cellulose fibers together. Theprocess by which lignin is removed is referred to as delignification.The most prevalent method of delignification is by chemical means inwhich raw wood chips and chemicals are combined at a controlled pressureand temperature in a vessel known as a digester. While in the digester,the amount of lignin removed from the wood chips determines the productquality, the product yield, the amount of energy consumed, the quantityof chemicals consumed, and the product cost. Fluid drained from thedigester during delignification contains lignin removed from the woodchips and is referred to as "black liquor". The black liquor is used asfuel in a boiler to produce process steam.

The measurement of the residual lignin remaining in the pulp, whichexits the digester, is most commonly carried out by laboratory analysisof hourly samples of the digester output (samples are typically obtainedat the last stage of the brownstock washer). The lab analysis takesapproximately one hour and therefore is a poor method for providingprocess control feedback and cannot be used for feedforward control.This lab analysis is a back titration method which measures theconsumption of the potassium permanganate and is only an approximationto the lignin concentration. The output of titration analysis isreferred to as "KAPPA Number" and the procedure is documented in TAPPIprocedure T236 hm-88, "KAPPA Number of Pulp[. A number of manufacturershave produced automatic sampling and titration devices which have beentried in pulp mill situations but they have been mostly unsuccessful inproviding accurate long term results and do not reduce the hour delaybetween the process and measurement of the residual lignin.

The ultraviolet absorption and fluorescence properties of lignin havelong been known and a number of researchers have reported results ofmeasurements in solutions containing lignin. Both the absorptiontechniques (e.g. see Kleinert, T. N. and Joyce, C. S., "Short WavelengthUltraviolet Absorption of Various Lignins and Related Substances," PartIV, Pulp and Paper Mag. Can. 58, Oct. 1957, pp. 147-152) and thefluorescence techniques (e.g. see Demas, J. N., Excited State LifetimeMeasurements, Academic Press, New York 1983) have all been applied tovery dilute solutions. The fluorescence techniques have been usedprimarily as a method of detecting trace quantities in effluent streams.All of these approaches made use of the very dilute lignin solutionswhere the absorption and fluorescence signal are linearly related tolignin concentration. The dilution is typically 2,000-10,000 times moredilute than the concentration of lignin in "black liquor" found in thepulping process and thus requires precise sample preparation prior tomeasurement. A number of devices which attempt to monitor the ligninconcentration in "black liquor" during the pulping process by UVabsorption techniques (alone or in combination with chemical analysis)have been produced. These devices require sample preparation anddilution prior to measurement and are therefore not in-situ, notreal-time, and introduce sampling and dilution errors. See Tikka, P. O.,and Virkola, N. E., "A New Kraft Pulping Analyzer for Monitoring Organicand Inorganic Substances", TAPPI Journal, June, 1966, pp. 66-71;Williams, D. J., "The Application of Ultra-Violet AbsorptionCharacteristic of Lignin to the Control of Pulp Uniformity", Appita,Vol. 22, No. 2, September, 1968, pp. 45-52; and Carpart, R.,Obese-Jecty, K., Le Cardinal, G. and Gelus, M., "Contribution of theOn-Line Kraft Pulping Control", PRP 4 Proceedings, Ghent, 1980.

Use of ultraviolet absorption has recently been extended to themeasurement of residual lignin in wood pulp (see Kubulnieks, E.,Lundqvist, S., and Pettersson, T., "The STFI OPTI-Kappa Analyzer,Applications and Accuracy", TAPPI Journal, November, 1987, pp. 38-42).The device disclosed in this article is marketed by Asea Brown Boveriunder the trade name "Opti-Kappa Analyzer". In this approach, the pulpstream is sampled approximately once every 5 minutes. The pulp sample isscreened, washed thoroughly, and diluted significantly. The dilutedsample is circulated in a loop where UV light absorption is measuredover a prescribed time period and the pulp concentration in the slurry(i.e., pulp consistency) is measured independently. This system involvessampling error, screening error, and pulp consistency measurement error.Although the system provides results much faster than the conventionallab titration process, it is still off-line. The washing requirements ofthis device are stringent since any small amount of black liquorremaining in the diluted solution will absorb UV light and produceerror. Bannier Technology Group (BTG Inc.) also offers a device whichoperates on similar principle but uses UV reflection rather thanabsorption. The BTG device is marketed under the name "KNA-5000 KappaNumber Analyzer".

All of the investigations and devices discussed so far used broad bandlamps as the source of UV light. In 1986, researchers at the NationalBureau of Standards (see Horvath, J. J., Semerjian, H. G., "LaserExcited Fluorescence Studies of Black Liquor," Proceedings of The SPIE,Vol. 665, June, 1986, pp 258-264) performed fluorescence tests ondiluted black liquor samples using a laser as the source of UV light.Although their investigation resulted in better signal-to-noise ratios,they essentially did not extend the art beyond that of previousinvestigators. They were only able to obtain a functional relationshipbetween fluorescence and lignin concentration in very dilute samples ofblack liquor (less than 1300 PPM, which is orders of magnitude less thanthe in-situ concentrations) and did not investigate pulp at all. Theydid not provide any insight into how one might be able to use either UVabsorption or fluorescence techniques to extend the useful measurementrange beyond the highly diluted state.

They did mention that this process was a candidate for in-situmonitoring but provided no rational explanation of how the dilutionrequirement could be overcome. They also mentioned that the measurementcould be made more acceptable for field use by using optical fibers toguide the UV excitation light to the process stream and carry thefluorescence signal back to the opto-electronics unit.

SUMMARY OF THE INVENTION

Based on a desire to meet the need for an on-line, real-time devicewhich could monitor the concentration of lignin in wood pulp and blackliquor, the present invention resulted from a project which examined thefluorescence of black liquor and wood pulp under excitation by variousnarrow band wavelengths of UV light. It is believed that these wood pulpexperiments were the first ever performed and the results are novel inthat a completely unexpected phenomenon was discovered. Namely, when theconcentration of lignin in the specimen is increased beyond the verydilute regime, which had been studied earlier by others, thefluorescence intensity levels off and then begins to decrease withincreasing concentrations of lignin. The region of most interest toon-line pulping is represented by a monotonically decreasing function offluorescence vs. lignin concentration. This monotonically decreasingfunction of fluorescence vs. concentration is known as the "quenchedfluorescence regime". Although the quenching phenomenon in molecularsubstances has been known for a long time, the shape of that curve,which can be flat, erratic, or decreasing, had never been empiricallydetermined for lignin containing substance prior to the presentinvention. This is important because the steep monotonically decreasingfunction discovered is not common and is the only curve that would makethe technique of the invention valuable in measuring lignin in theundiluted product.

It has also been found that the fluorescence signal which is produced inundiluted wood pulp, as it flows past the last brownstock washer in thepulp mill, is unaffected by trace amounts of black liquor remaining inthe pulp at that stage of the process. This means that the measurementcan be made on-line without having to wash the pulp beyond the levelalready performed in the normal pulping process.

The invention also includes three technical enhancements which improveaccuracy and resolution of the measurement. These three enhancementsare:

A. Use of more than one UV excitation wavelength to discriminate betweenthe fluorescence of lignin and any potential interferents.

B. Use of time resolved fluorescence to eliminate unwanted fluorescenceand to make the functional relationship between fluorescence and ligninconcentration even more steep, thus resulting in improved resolution inhighly concentrated substances.

C. Use of phase resolved fluorescence to eliminate the unwantedfluorescence.

The invention also uses mechanical distancing, special optics, andproximity sensors to make possible the measurement of a moving pulp matwhose distance from the probe is varying. For the measurement of pulpand/or black liquor in a pipe line the invention also uses a number ofprobe configurations.

The invention further includes the results of investigating variableexcitation wavelengths, phase resolved fluorescence, and time resolvedfluorescence. All of these methods have been successful indiscriminating the fluorescence of lignin in the presence of otherfluorescent species in undiluted wood pulp.

Accordingly, an object of the present invention is to provide anapparatus for and a method of monitoring lignin concentrations in woodpulp and black liquor on a real-time, on-line basis.

A further object of the the invention is to provide an apparatus formonitoring lignin concentration which is simple in design, rugged inconstruction and economical to manufacture.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich the preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a graph plotting fluorescence vs. lignin concentration asmeasured by pulp KAPPA Number in pulp monitored according to the presentinvention using continuous excitation at 334 nm, which demonstrates thefunctional relationship at undiluted lignin concentrations;

FIG. 1A is a graph plotting light intensity against time for anexcitation light pulse and resulting fluorescent behavior of fluorescingmaterial in general;

FIG. 1B is a graph plotting fluorescence intensity against time fordifferent fluorescing species;

FIG. 2 is a schematic block diagram of a lab set up used in accordancewith the present invention using continuous excitation;

FIG. 3A is a composite graph plotting fluorescence intensity vs. ligninconcentration (PPM) discovered using the lab set up of FIG. 2 on avariously diluted black liquor sample (BL5) at an excitation wavelengthof 334 nm;

FIG. 3B is a composite graph plotting fluorescence intensity vs. ligninconcentration (ppm) discovered using the lab set up of FIG. 2 on avariously diluted black liquor sample (BL3) which si the same as (BL5)with expanded axis;

FIG. 4 is a block diagram of an apparatus used to measure ligninconcentration in a black liquor sample, using phase resolvedfluorescence;

FIG. 5 is a view similar to FIG. 4 of an apparatus for measuring ligninconcentration on a time resolved basis;

FIG. 6(a) is a schematic diagram of an apparatus used for measuringlignin in a sample with dual excitation wavelength fluorescencespectroscopy;

FIG. 6(b) is a frontal view of chopper wheel (51);

FIG. 7(a) is a plot of fluorescence intensity versus KAPPA Number withexcitation at 337 nm;

FIG. 7(b) is a plot of fluorescence intensity versus KAPPA Number withexcitation at 488 nm;

FIG. 8 is plot of response vs. Kappa number combining measuredquantities, F (337) and F (488), excitation at 337 nm and 488 nm versusKAPPA Number;

FIG. 9 is a partial schematic view of a device according to the presentinvention for maintaining a selected distance between a probe used inaccordance with the present invention and a pulp drum carrying a layerof pulp;

FIG. 10 is a view similar to FIG. 9 of another embodiment of the device;

FIG. 11 is a view similar to FIG. 9 of a still further embodiment of thedevice;

FIG. 12 is a view similar to FIG. 9 of a still further embodiment of thedevice;

FIG. 13 is a front elevational view of a device for maintaining aselected spacing between a pulp mat and a probe used in accordance withthe present invention;

FIG. 14 is a schematic side view of a still further embodiment of theinvention for measuring lignin concentration on a pulp mat;

FIG. 15 is a view similar to FIG. 14 showing a further embodiment of thepresent invention;

FIG. 16 is a view similar to FIG. 14 showing a still further embodimentof the invention;

FIG. 17 is a view similar to FIG. 14 of a still further embodiment ofthe invention;

FIG. 18 is a side elevational view of a probe and flow tube combinationfor measuring the lignin concentration in pulp slurry or in black liquoraccording to the present invention;

FIG. 19 is a view similar to FIG. 17 of a different embodiment thereof;

FIG. 20 is a view similar to FIG. 17 of a still further embodimentthereof;

FIG. 21 is a view similar to FIG. 18 of a further embodiment of thepresent invention;

FIG. 22 is a schematic block diagram of a laboratory set up forverifying the excitation wavelengths and time resolved fluorescencetechniques of the present invention; and

FIG. 23 is a plot of fluorescence intensity vs. KAPPA Number using timeresolved fluorescence at 12 ns delay using the device of FIG. 22.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings in particular, the invention embodied thereincomprises a method and apparatus of monitoring lignin concentration inwood pulp or black liquor, which takes advantage of the predictable andreproducible fall off in fluorescence as lignin concentration increases,illustrated in the graph of FIG. 1. This portion of the curve isreferred to as the quenched fluorescence regime. FIG. 1 shows theunwashed specimen test results (RES1) with an excitation wavelength of334 nm.

Referring to the graph shown in FIG. 3A, the quenched side beginsimmediately after the peak on the curve and continues to includeeverything to the right side of the curve therefrom. At an excitationwavelength of 334 nm and other nearby wavelengths, the ligninconcentration expressed in KAPPA Number falls off in such a predictablemanner that fluorescence intensity can be utilized to calculate ligninconcentration.

Excitation wavelengths less than 500 nm can be used to excite ligninfluorescence. Shorter wavelengths are preferred since they producestronger signals and are more selective than longer wavelengths. Theactual wavelength chosen depends upon the embodiment. Results have beenobtained using the 313 nm, 334 nm, and 365 nm lines (wavelengths) of aMercury arc lamp, the 325 nm line of a HeCd laser, the 337 nm line of apulsed N₂ laser, and 360-500 nm wavelengths of a pulsed dye laser.

FIG. 1A illustrates the temporal behavior of the fluorescing materialwhen excited by a pulse of light (2) having short duration. During theexcitation pulse, the fluorescence intensity (4) rapidly. After theexcitation pulse, fluorescence begins an exponential decay with acharacteristic and identifiable time constant.

FIG. 1B shows the fluorescence from two different species (A,B) underpulsed excitation conditions. The decay time constant of species A ismuch longer than that of B. If a sample to be analyzed according to thepresent invention contains both species, the resultant signal will bethe sum of the two curves in FIG. 1B.

In a conventional, non-time resolved approach, one would generate asignal proportional to the total area under the resultant curve. Inother words, instead of looking at how the signal changes with time, onewould merely integrate the total signal over the total time offluorescence.

When the sample contains only one or the other of the components A andB, this type of signal is sufficient to provide a measure of theconcentration of that component. If both components are present, onecannot separate the contributions of the two and, hence, cannotdetermine the concentration of either.

In time resolved fluorescence spectroscopy the invention makes use ofthe additional information that component B fluorescence decays muchmore rapidly than does that from A. If the decay time difference islarge, one can simply wait to turn the detector on until virtually allof the fluorescence from B is gone. The remaining signal, althoughsmaller than the total, can then be attributed to A and therefore usedto find the concentration of A. Knowing this concentration of A, the Bconcentration can then be calculated from the conventional "all time"measurement.

In fact, the difference in decay times need not be so large as to allowthe complete decay of B before making the measurement. As long as thetime constants are known, any two measurements made over two differenttime intervals, provide the information necessary to calculate theconcentration of both A and B.

Phase sensitive spectroscopy of the invention is based on the sameeffect, e.g., the differential in decay times. Instrumentally, however,it is altogether different. Instead of pulsing the excitation and makingmeasurements at known time intervals after the pulse, as in the timeresolved approach, a continuous source is used. This source is thenrapidly modulated. This in turn modulates the fluorescence signal. Inother words, if one turns the excitation on and off at some rate, thefluorescence signal will turn on and off at the same rate. Because ofthe time constant, however, the fluorescence signal does not shut off atthe same time as the source but at some time later determined by thedecay time constant. The fluorescence signal then has the same frequencyas the source modulation but is delayed in phase, the phase delay beingproportional to the decay time constant. When the sample has two or morecomponents, the fluorescent signal is a sum of two or more signals allhaving the same frequency but each differing in phase. These signals canbe electronically separated on the basis of this phase difference andused separately to determine the concentration of individual components.

The present invention can thus distinguish the fluorescence due tolignin even when other fluorescing materials are present in the sample,as long as the spectral characteristic of each material is known inadvance.

FIG. 2 illustrates an apparatus of the present invention for collectingfluorescence intensity data which comprises a light source (10) in theform of a mercury arc lamp. Lamp (10) shines light through an opticalsystem in the form of a lens (12), a first filter (14) and a second lens(16) which focuses the light onto a sample cell (18) containing pulp orblack liquor. Fluorescent light emitted from sample cell (18) passesthrough a second filter (20) and along a fiber optic bundle (22), to amonochromator (24). A light intensity detector (26) such as "SIT"detector is connected to the output of monochromator (24) to generate asignal which is processed in circuitry (28). Circuitry (28) is connectedto display means (30) which produces a graphic representation offluorescent intensity plotted against wavelength showing an excitationpeak (6) and an emission peak (8). For an excitation wavelength of 334nm, the concentration of solids in black liquor, and thus theconcentration of lignin, in sample cell (18) is changed to produce theresults illustrated in FIGS. 3 A and 3B for a solids concentrationapproaching 0 up to a concentration of about 650,000 parts per million(ppm). The concentration of lignin in the black liquor is roughly 50% ofthe solids concentration. Although fluorescence increases up to about13,000 ppm, it thereafter drops off as shown in FIG. 3A.

FIG. 4 schematically illustrates an apparatus which can be utilized forpracticing the phase resolved version of present invention off-line. Foron-line operation, one of the probe configurations shown in FIGS. 9-21may be incorporated. The apparatus comprises a probe (40) attached todepth adjusting means (42) for moving the detecting end (44) of theprobe closer to or further away from a sample (48) for example a woodpulp mat, held on a precision XY translation table (50) which can changethe relative position of the mat (48) to the detecting end of the probe(40). Probe (40) comprises a central excitation tube (52) having aband-pass filter (54) at its lower end for passing a selectedwavelength, such as 334 nm, of excitation light. Light is supplied tothe excitation tube (52) by excitation optical fibers (56) attached at acoupler (58) to an excitation source generally designated (60).Excitation source (60) has a light source (62) powered by a power supply(64) for passing light through an optical arrangement with a cold mirror(31) and heat sink (33) including an electric shutter (66) which iscontrolled by an input/output (I/O) device (68) connected to amicroprocessor (70) in a system processor arrangement (72). Lightamplitude modulator (37) modulates the light source (62) and signalgenerator (43) establishes the frequency of modulation. Driver (46)amplifies the output signal of the signal generator in a known manner.Other lenses and filters, for example, lenses (34), (36), verticalpolarizer (38), and bandpass filter (39), are provided in excitationsource (60) for conditioning the light supplied through filter (54) ontothe sample (48). Fluorescent light from the sample (48) is conductedthrough a long pass filter (32) and a tube (53) forming another part ofprobe (40). This light is conducted along an optical fiber bundle (57)to an opto-electronic package (74). Coupler (58) also provides areference light source by way of referenced fiber (59) to theopto-electronics package (74). Chopper (35) supplies the referencesignal to lock-in amplifier (80). The opto-electronics means or package(74) includes the following components for both sample and referencesignals: filters (71, 71'), detectors (73, 73'), and amplifiers (82,84). The reference light is supplied to a phase shift element (76) andthen compared with respect to phase, to the fluorescent light in acorrelator (78). The correlation or lack thereof between the source andfluorescent light is applied to the microprocessor (70) through alock-in amplifier (80). Microprocessor (70) is programmed with theinformation necessary to calculate lignin concentration from phase shiftinformation, the phase shift information corresponding to thefluorescent light intensity due to fluorescent lignin in the sample. Thesignals from microprocessor (70) can also be utilized to move theshutter (66) and the XY translation table (50) for taking a freshreading.

FIG. 5 is an embodiment similar to FIG. 4 for measuring the intensity ona time resolved basis. The same reference numerals are utilized todesignate the same or functionally similar parts. Where the parts havealready been described in connection with FIG. 4, the description willnot be repeated.

The time resolved embodiment of FIG. 5 utilizes a pulsed laser (63)operating at a selected wavelength such as 337 nm which shines lightthrough a beam splitter (67), to the optical fiber bundle (56) carryingthe excitation light. A fiber bundle (69) conveys the divided part ofthe split beam from laser (63), to a high voltage pulse generator (61)which applies gating pulses to a pair of high speed detectors ordetector amplifiers (82, 84) in opto-electronics package (74) which maycontain a monochromator at the asterisk. The amplifiers (82, 84)respectively receive pulses proportional to light intensity on emissionfibers (57), corresponding to the fluorescent intensity from the ligninin sample (48), and an optical fiber bundle (59) which supplies pulsedlaser light from the coupler (58). The opto-electronics package (74)thus provides time resolved comparisons between excitation andfluorescent light of sample (48), which is processed in microprocessor(70).

FIG. 6(a) is another embodiment similar to FIG. 5 for measuring thelignin concentration in wood pulp using dual excitation wavelengthfluorescence spectroscopy. The same reference numerals are utilized todesignate the same or functionally similar parts. Two lasers areoperated at two different wavelengths, λ₁, and λ₂, for example λ₁ =337nm and λ₂ = 488 nm. Of course, there is great flexibility in selectingexcitation wavelenghts. The two laser beams λ₁ and λ₂ are combined by amirrored chopper wheel (51) rotated by a stepper motor (55). The chopperwheel (51) consists of a series of open slots (51a) alternating withmirrors (51b) as best seen in FIG. 6(b). When the open slot (51a) is atthe point of intersection of the two beams, λ₁ and λ₂, only λ₂ passesthrough and is input through a lens (13) to the source or excitationoptical fiber or fiber bundle (56) where it is received and transmittedby probe (40) to a sample (48) such as a pulp mat.

Laser beam λ₁ passes to the excitation fiber (56) only when the mirror(51b) is at the point of intersection. While laser beam λ₁ is directedto probe (40) by way of lens (13) and the excitation fiber (56), laserbeam λ₂ is blocked by mirror (51b) of the chopper wheel (51). In thisfashion, the laser beam entering the excitation fiber (56) alternates intime between the two excitation wavelengths.

Probe (40) focuses the laser beam on the sample (48) with the sample'semitted fluorescence being collected by the same. Emission or detectorfiber or fiber bundle (57) carries fluorescence signals to a detector(74) as previously described with respect to FIGS. 4 and 5. In thisembodiment the signal from detector (74) is sent to a system processorarrangement (72) and alternates between that of λ₁ and λ₂. During theinitial calibration and set-up, a weighting constant, C, may be set to adesired value simply by adjusting the intensity of laser beam λ₂. Thesignal from detector (74) will then be a square wave whose amplitude isthe desired function, e.g., F(λ₁)-C*F(λ₂).

The total fluorescent emission from undiluted pulp samples can beexpressed as follows:

    F=Af.sub.1 +Bf.sub.2                                       (I)

where:

f₁ =the fluorescence that correlates well with the concentration oflignin as measured by the standard wet chemical method and expressed asKAPPA Number.

F₂ =the fluorescence that does not correlate well with measured ligninconcentration.

A and B are constants.

Because of the f₂ component, the correlation between F and Kappa Numberis poor, as shown in FIGS. 7(a) and (b), and F cannot be used as ameasure of lignin concentration. The data in FIGS. 7(a) and (b) wereobtained with a device schematically depicted in FIG. 6(a) set at λ₁=337 nm and λ₂ =488 nm. A comparison of FIG. 7(a) with FIG. 7(b) showsthat the relative effect of the f₂ component is greater when thefluorescence is excited by a longer wavelength source, i.e., 488 nm.

Therefore, equation (I) can be rewritten as follows:

    F(337)=A(337)*f.sub.1 +B(337)*f.sub.2                      (II)

    F(488)=A(488)*f.sub.1 +B(488)*f.sub.2                      (III)

When these two equations (II) and (III) are combined to eliminate the f₂term, the following equation results: ##EQU1## where: C and D areconstants that are combinations of the original A's and B's.

Since f₁ provides a good measure of the lignin concentration asexpressed by KAPPA Number, the two measured quantities, F(337) andF(488), are used to calculate the relative lignin concentration providedthe constant C is known. Empirically, it was determined that theconstant C has the value of 0.29 for investigated pulp samples. FIG. 8shows that combining the measurements in this way gives a well behavedmonotonically decreasing function suitable for the determination ofKAPPA Number. It remains to be seen if the constant, C, is the same forpulps from different woods and/or different processes. If not, acalibration is simply required to determine this value for a given typeof pulp.

FIG. 9 shows a mounting for probe (40) in a vacuum sleeve (90) which isengageable at a selected distance from a pulp mat on a pulp drum (92).The signals from probe (40) can be processed in the apparatus of FIGS.4, 5 and 6(a) to monitor lignin concentration in the pulp mat.

FIG. 10 shows a contact version of the invention where a probe (40) isin contact with the mat through a standard thickness transparent layer(94) in contact with the mat on the drum (92).

FIG. 11 utilizes probe (40) which is fluid coupled at (41) to thesurface of a silica (SiO₂) cylinder (96) in contact with the pulp mat ondrum (92).

FIG. 12 shows an embodiment where the probe (40) is mounted as a spokeon a quartz cylinder (98) in rolling contact with the mat on drum (92).

In the embodiment of FIG. 13, the sensing end (44) of probe (40) is heldat an accurate and selected distance from pulp mat (48) by mounting theprobe (40) on the axle (100) of a pair of rollers (102) rolling againstthe pulp mat (48).

FIG. 14 illustrates a non-contact pulp mat probe arrangement where thepulp mat (48) is illuminated by a light source (62) with fluorescentlight being received by light sensor (104), for processing.

In the embodiment of FIG. 15, probe (40) both shines and receives lightthrough a lens (106) to and from the mat (48).

In the embodiment of FIG. 16, mat (48) is illuminated by a probe (40)having an outer light source (108) and a central fluorescent responsetube (110). A lens (107) having separate inside and outside elements forshining and receiving the light is provided between the probe and thepulp mat.

In the embodiment of FIG. 17, probe (40) is set at a known desireddistance from mat (48) by a proximity sensor (112) such as an ultrasonicdistance instrument which is physically connected to the probe. In thenon-contact version of the present invention, maintaining a set andaccurately known distance between the probe face and the mat isessential to avoid variations in light intensity which, rather than dueto lignin concentration, is due to distance variations.

Embodiments of the invention for measuring lignin concentration in blackliquor or pulp slurries are shown in FIGS. 18-21.

FIG. 18 shows probe (40) which may be the same design as the probesutilized in the equipment of FIGS. 4, 5 and 6(a) engaged to an aperturein a flow tube (114) which contains a flow of black liquor or pulpslurry.

In the embodiment of FIG. 19, probe (40) penetrates tube (114) in arecess (115). The sensor face of probe (40) is serviced by a fluidinjector (116) which may be used to scour and clean the sensor face.

A similar injector (116) is used in a recess (115) of the tube (114) inthe embodiment of FIG. 20 where probe (40) is mounted next to multiplewindows (120) which are used to insure the presence of black liquorflow. A single long window may replace the two windows (120).

FIG. 21 shows an embodiment of the invention where flow is normallyconducted through a supply valve (122) downstream of a Y-connection(123) in the flow pipe (114). When a real-time measurement is to betaken, valve (122) is closed and a second valve (124) is opened whichcauses the stream to pass probe (40). The streams are reconnected at asecond Y-connection (125).

FIG. 22 illustrates an apparatus for verifying the usefulness of theinvention which comprises a nitrogen laser (130) which supplies light toan optical arrangement of lenses and mirrors (132), to a sample 48mounted on a translator (134) and to a fiber (136) which supplies areference light signal as a trigger to electronic sensing equipment.Fluorescent light is supplied over a fiber bundle (138) to the input(139) of a monochromator (140). The output (142) of monochromator (140)is supplied to a detector (144) such as an IRY-690G/B/Par detector forexample.

The apparatus of FIG. 22 was utilized to measure fluorescence at timedelays of 00, 04, 06, 08, 10 and 12 nanoseconds (ns) to reveal thecorrelation between fluorescent light intensity and KAPPA Number. FIG.23 is a plot of fluorescent intensity versus KAPPA Number using timeresolved fluorescence at 12 ns delay with the device shown in FIG. 22.The time delay (T₀) equals 12 ns to a final time (T₁) of 2,000 ns withwavelength integration of 360 to 700 nm and excitation at 337 nm.

While the specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. An apparatus for measuring lignin concentration with time resolved fluorescence in an undiluted wood pulp or black liquor sample, on a real-time, in situ basis, comprising:light source means for applying excitation light pulses at a selected wavelength and at known time intervals to the undiluted sample for causing the lignin concentration to produce fluorescent emission light with a fluorescence intensity that monotonically decreases in a quenched fluorescence regime; light detector means for measuring the emission light at the known time intervals and establishing signals indicative thereof; switching means for turning said light detector means on at precise specified time intervals after each excitation light pulse; and signal processing means connected to the light source means and the light detector means for comparing intensities of the emission light from the lignin in the quenched fluorescence regime to the intensities of the excitation light pulses on a time resolved basis for providing a measurement of the lignin concentration in the undiluted sample as a function of the time resolved emission light intensity.
 2. An apparatus according to claim 1, including a probe having a sensing end for facing the sample, said probe being connected to the light source means for conveying the excitation light pulses to the sample, said probe being connected to said light detector means for conveying emission light from the sample.
 3. An apparatus according to claim 2, including means for mounting the probe at a selected distance from the sample.
 4. An apparats for measuring lignin concentration with phase resolved fluorescence in an undiluted wood pulp or black liquor sample on a real-time, in situ basis, comprising:light source means for applying continuous excitation light at a selected wavelength to the undiluted sample for causing the lignin concentration to produce fluorescent emission light with a fluorescence intensity that monotonically decreases in a quenched fluorescence regime; means for modulating the continuously applied excitation light; light detector means for measuring the modulated emission light of the lignin in the quenched fluorescence regime and establishing signals indicative thereof; a reference light provided to the light detector means modulated in accordance with the modulated applied excitation light; and signal processing means connected tot he light detector means for correlating intensity with respect to phase of the modulated emission light to the intensity of the modulated excitation light for providing a measurement of the lignin concentration in the undiluted sample on a real-time, in situ basis as a function of the phase resolved emission light intensity.
 5. An apparatus according to claim 4, including a probe having a sensing end for facing the sample, said probe being connected to the light source means for conveying the modulated excitation light to the sample and said probe being connected to said light detector means for conveying modulated emission light form the sample.
 6. An apparatus according to claim 5, including means for mounting the probe at a selected distance from the sample.
 7. An apparatus for measuring lignin concentration in an undiluted sample on a real-time, in situ basis, comprising:first and second light sources for supplying excitation lights at first and second selected and different wavelenghts to the undiluted sample for causing the lignin concentration to provide fluorescent emission light with a fluorescence intensity; means for selectively alternating transmission of the first and second wavelenghts of excitation light to the undiluted sample; light detector means for measuring the emission light of the lignin on the quenched side for both wavelengths and establishing first and second signals indicative thereof; signal processing means connected to the light detector means for measuring the signals of the emission light at both wavelengths and comparing and signals to the intensity of the excitation light for both wavelengths; and means for converting the compared signals into a function that monotonically decreases in a quenched fluorescence regime to provide a measurement of the lignin concentration in the sample that uses the emission light intensity for both wavelengths.
 8. An apparatus according to claim 7, including a probe having a sensing end for facing the sample, said probe being connected to the first and second light sources for conveying the excitation light at both wavelenghts to the sample, said probe being connected to said light detector means of conveying emission light from the sample.
 9. An apparatus according to claim 8, including means for mounting the probe at a selected distance from the sample.
 10. An apparatus according to claim 7, wherein said fist and second light sources include two lasers. 